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I was looking at some pvc feeder designs today...  We've got a bunch left over from when we built our house, all we would need would be the joints/connetors. I'd been thinking about floats too.  where do you get that type and what kind of costs on them?
for an automatic waterer, maybe use pvc, cut holes or slot lengthwise in the top and put the float at one end?  
I've seen people mention putting it above the level of their backs or even high enough that they have to reach for it, so that may be an option too?  
if it's mounted against a wall then they wouldn't be likely to perch on it either.  
personally though, if you're using a float, i'd keep the waterer as small as possible for easier cleaning when it does get dirty.  maybe use a short piece (2' or less) of 4" or 6" pipe, with holes cut along the top for them to drink.  about 12" of drinking space plus whatever the float requires would probably do for the few birds you're planning.
I'm not going to create a new image to show what i'm thinking, but here's the idea i posted about the pvc automatic waterer.  for the float just use the horizontal part of the one on the right...  you get the idea (i hope).
https://www.backyardchickens.com/forum/uploads/88409_pvc_water.jpg
Looking forward to seeing the completed coop.  8)  you've got my hubby contemplating our next coop now.  
Karen - k.i.forgot
i acyaully was thinking of the same thing.. I want to install a 2" PVC and cut oblong holes along the top. and perhaps have it connected to a box where the float would be. I just want it to be able to clean it without problems...
those floats i picked up on 
ebay under aquarium tanks auto fills  $6. ea.  I may spare a couple if you are interested.. I believe  still have about 6 in the garage.. Still new in a box..